The present invention relates to a boost-converter with low losses.
So called boost-converters are normally used in order to create a higher second dc-voltage from a generated first voltage, which second voltage is fed to some kind of load. Such a converter normally makes use of a series connection of an energy storing inductor and a diode connected between an input-terminal from the first voltage source and an output-terminal for the second voltage. The centre-point between the series connected components is connected to earth via a switching device as such a transistor. A capacitor can be connected between the input-terminal and earth in order to decouple current ripples on the input and to damp radio interference to the fed power supply system. In addition, an energy storing (filtering) capacitor is connected between the output-terminal and earth. Finally, a control circuit for the switching element is arranged to monitor the output voltage from the converter and to control the switching element so that, for example, the output voltage is kept constant independent of the load and the input voltage for a given period.
A simple boost-converter thus functions in the following manner. When the switching element switches to non-conducting condition, the voltage at the centre point increases to a value which exceeds the input voltage. The diode conducts in the forward direction and current is fed to the load including the capacitor at the output. When the switching element thereafter switches to conducting condition due to a signal from the control circuit, current from the centre point will be conducted to earth via the switching element, whereby the voltage at the centre point falls. The current through the switching element is, in the first instance, composed of a contribution from the input, via the inductance, and a contribution which is in the form of a reverse current through the diode which has not yet been blocked. After a certain delay, the recovery time of the diode, the diode is blocked and the current through, just as with the voltage across, the switching element rapidly decreases.
The reverse current phenomena through the diode is a problem with this type of circuit and creates power losses in both the diode itself and the switching element. Because of strong current spikes, radio interference can also arise.
It is generally desirable to operate the device with a high switch frequency in order to limit the inductance value and the physical size of the inductor. If no measures were taken to restrict the reverse current in the diode, the power losses would however increase with increased frequency and the efficiency of the circuit would accordingly decrease.
Higher output-voltages also accentuate the problem since diodes for higher voltages generally have longer recovery times.
In order to reduce the size of the reverse current, it is previously known to place an inductor between the diode and the centre point.
In American patent U.S. Pat. No. 4 591 966 there is for example shown a connection in a boost-converter in which the reverse current through the diode, and thereby the switching element (the transistor), is restricted using a saturable inductance connected in series with the diode. The circuit also comprises means to bring the inductor to a saturated condition when the transistor is switched off so that low impedance is obtained for the current in the forward direction of the diode. Using this arrangement, a restriction of the reverse current in the diode is achieved and thus the associated power losses in the diode and transistor when the latter is conducting.
A second solution to the same problem is described in European patent application EP-A-351 144 in which, in addition to the saturable inductor in series with the diode, a second inductor element is provided through which the reverse current passes during restriction of said current. Energy stored in the inductor element is used when the transistor interrupts the current path to earth to bring the inductor in series with the diode to a saturated condition in order to conduct the current in the forward direction of the diode. Using this method it is possible to reduce the power losses in the circuit.